Generic methods are well known in prior art. A flaw detected in the volume of the test object by means of an pulse echo method based on insonifying pulsed ultrasound into the test object, for example, a cavity, an inclusion or also a crack, are characterized by specifying a value for its equivalent reflector size ERS. The value of this equivalent reflector size is determined by a comparison of the amplitudes of the echo signals that are caused by the tested flaw in the volume of the test object, with the model of a comparative flaw of known size. In the so-called reference body method, the test operator compares the echo signals of the tested test object with the echo signals which he obtains on a reference body that is equivalent to the test object, which has one or more reference reflectors. For example, for this purpose, cylindrical bores with known dimensions can be inserted into the reference body. The echo signals occurring at the bore during an ultrasound reflection are then compared with echo signals that are obtained while testing the test object. In the reference body method, the test operator therefore uses a suitable test probe, that can be, for example, a suitable angle test probe, to take measurements both of the test object that is to be tested, as well as of the prepared reference standard.
In contrast, in the so-called DGS method, the amplitude of an echo signal resulting from a flaw in the volume of the test object is compared with a theoretically calculated and/or empirically determined echo signal of a model reference flaw, which is assumed to be a plane circular disk, as a rule, and which is at the same depth in the test object as the flaw detected during the test of the test object. For this purpose, a so-called DGS diagram is prepared in advance for the test probe used in the test, which contains the characteristics of the test probe. The curves contained in the DGS diagram indicate the echo amplitude that would be produced by a reference flaw in a measurement with the test probe that is used. In a practical testing task, the test operator can then read the equivalent reflector size of the flaw detected in the volume of the test object by carrying out a sound-attenuation correction (material-specific sound attenuation) and transfer correction (test object-specific coupling losses) for the test object directly off the DGS diagram.
In a classic test method according to the DGS method, the test operator varies the test probe position and orientation relative to the flaw found and tries to thereby maximize the resulting echo signal. This process is also described as “breeding” the ultrasound signal when testing materials by means of ultrasound. The actual determination of the equivalent reflector size of the detected flaw then takes place for the maximized ultrasound echo.
Additional details of the DGS method result, for example, from patent specification U.S. Pat. No. 5,511,425 A, which goes back to the legal predecessor of the applicant. Furthermore, the DGS method is described in detail in the book “Material Testing with Ultrasound”, J. Krautkrämer and H. Krautkrämer, 5th edition, Springer Verlag, ISBN 3-540-15754-9, chapter 19.1, pages 343-349. The technical details concerning the DGS method disclosed here are being added in their entirety to the content of the disclosure of this application by way of this reference.
In its currently prevalent form, it is a disadvantage of the DGS method that, for a meaningful characterization of a flaw in the volume of a test object, a test must be performed with a plurality of test probes. This is due to the fact that, for a given flaw, a perpendicular insonification into the test object does not necessarily supply a maximum echo amplitude. Rather, it depends on the orientation of the flaw in the volume of the test object under which insonification angle a maximum echo signal can be obtained. In order to actually obtain a value for the equivalent reflector size of a detected flaw that is reasonably correlated with the actual size of the flaw, therefore, as a rule, different angle test probes are used within the context of standardized test procedures based on the DGS method, which realize different insonification angles. In practice, this method signifies a considerable testing and documentation effort for the test operator, so that as a rule, testing is only performed under a few insonification angles. Moreover, the variation of the insonification angle requires a change of the test probe, which causes additional problems because of the calibration, which is never absolutely unequivocal, and the coupling properties of the test probes. This also renders the interpretation of the ERS values found on a flaw more difficult.